A serious problem exists in the photolithographic arts. Surprisingly enough, this problem is that the existing high-resolution optical inspection systems (i.e. optical microscope) can detect almost every defect on a wafer. This is problematic because it is not always necessary to detect all defects on an inspection surface at all times. For example, during a photomask fabrication process, the ability to detect all defects is important, but the same may not be true always true during associated wafer production processes. Still worse, the present art has no efficient method of separating the unimportant defects from the lithographically significant defects. Currently, it requires massive amounts of time and effort to separate the important and unimportant defects. The inventors have developed methods and apparatus for vastly reducing this time and for more efficiently sorting out the lithographically significant defects.
Integrated circuit fabrication utilizes photolithographic processes which use photomasks or reticles and a projection optical system (e.g stepper or scanner) to project circuit images onto silicon wafers. A high production yield is contingent on having defect-free masks and reticles. Since it is inevitable that defects will occur in the mask, these defects must be found and repaired prior to using the mask.
Automated mask inspection systems have existed for over 20 years. The earliest such system, the Bell Telephone Laboratories AMIS system (John Bruning et al., “An Automated Mask Inspection System—AMIS”, IEEE Transactions on Electron Devices, Vol. ED-22, No. 7 July 1971, pp 487 to 495), used a laser that scanned the mask. Subsequent systems used a linear sensor to inspect an image projected by the mask, such as described by Levy et al. (U.S. Pat. No. 4,247,203, “Automatic Photomask Inspection System and Apparatus”). Such a technology teaches die-to-die inspection, i.e., inspection of two adjacent dice by comparing them to each other.
As the complexity of integrated circuits has increased and the size of features has decreased, so have the demands on the manufacturing and inspection processes. Many tools and approaches have been developed to address this need for accurate defect detection. Commonly, mask inspection is performed using high resolution magnifying optical inspection systems. Such systems use high magnification optical systems to project mask reticle images onto sensors to enable defect detection and analysis. These devices have high numerical aperture (NA) in the imaging plane. These devices have become so effective at detecting defects that mask inspections can reveal thousands of defects on a single mask. Unfortunately, this requires that each defect be individually examined to gauge its lithographic significance in the final printed pattern. Needless to say this can take a very long time and using current technologies and defect modeling is a very error prone process. Consequently, such inspections generate a significant process bottleneck.
FIG. 1A provides a simplified schematic depiction of an existing photolithographic patterning apparatus 100. One example is a stepper apparatus configured to conduct pattern transfer between a mask reticle to a substrate (commonly, a photoresist layer on the substrate). Commonly, an illumination source 101 directs a light beam into the illumination optics 102 of the stepper 100. The illumination optics 102 here are represented by the extremely simplified depiction of FIG. 1(a). The illumination optic system 102 can include, for example, collimating optics 102a, an aperture 102b, and focusing optics 102c all integrated to form an illumination beam 103 that is directed onto a selected portion of a mask reticle M. The mask, of course, affects the beam to form a patterning beam 104 that is directed into projection optics 105 which focuses the beam onto a selected location of a target 106. For such a reduction optical system, the ratio between the image NA (numerical aperture) at 105i and the object NA at 105o is a reduction factor (for example, a 4× reduction factor is common). It is pointed out, that in order to achieve high-resolution pattern transfer and 4× reduction in size, projection systems employ projection optics 105 having a high image NA 105i and a low object NA 105o. For the purposes of this disclosure, a high NA generally has a value greater than 0.85 and more particularly greater than about 1.0. Thus, in the system of FIG. 1 the high image NA 105i has an NA of about 0.85 or greater. These NA's can also be enhanced using immersion fluids. In contrast, the object NA 105o on the same systems are generally above 0.20. For example, to achieve a 4× magnification on a system with an image NA 105i of about 0.85, an objective NA of about 0.2125 ( 0.85/4) is used. Similarly, for a system with an image NA of about 1.00 an objective NA of about 0.25 can be employed.
However, when inspections are performed, systems having different optical parameters are employed. Detectors (CCD's and the like) are commonly formed with pixel sizes in the 10 micron range. Thus high magnifications are required so that the detectors can image the very small sub-micron features of the mask. For example, inspection tools commonly have a relatively high object NA coupled with a low image NA in order to obtain the required degree of magnification. Additionally, because mask features are continually decreasing in size (in the range of 10's of nanometers) increasing magnifications are required. Thus, magnifications on the order of 200× or more (and higher) are often used to conduct inspections.
FIG. 1B illustrates an example prior art inspection tool 200 showing the need for high magnification optics 207 having small image NA's 107i. An illumination source 201 directs a light beam into the illumination optics 202 of the inspection tool 200. The illumination optics 202 here are represented in an extremely simplified depiction in FIG. 1B. The illumination optic system 202 can include, for example, collimating optics 202a, an aperture 202b, and focusing optics 202c all integrated to form an illumination beam 203 that is directed onto a selected portion of a mask reticle M. The parameters of the illumination optical system 202 are subject to a wide range of variability. The mask M, of course, affects the beam to form an inspection pattern beam 204 that is directed into magnifying optics 207 which focuses the beam onto a detector element 208 where it produces a signal corresponding to the received light. It is pointed out, that for achieving high magnification, generally a high object NA at 107o and low image NA at 107i are employed. Typically, a high object NA for an inspection tool is in the range of about 0.5-0.9, and the corresponding low image NAs are below 0.01. Such lens systems give suitable magnification.
Such mask inspection and defect detection tools are very efficient at locating defects. Given the current state of technology an inspection can reveal the number of defects in the range of hundreds to thousands or more per mask. During a photomask fabrication and inspection process, finding all those defects is generally considered desirable and beneficial. However, during wafer production processes, it is sometimes desirable to detect only those defects that have a lithographically significant impact on the actual photolithographic pattern. Unfortunately, current technologies require that each defect be individually examined to gauge its effect of the photolithographic pattern. Worse still, examining each of the thousands of defects is so time-consuming as to be prohibitive. This is doubly troublesome because many defects discovered are relatively unimportant and have little or no effect of the pattern as printed. Regrettably, given the current state of the art, it is difficult, if not impossible to determine if a given defect is important or not. So, currently all defects must be subject secondary inspection and verification of their significance.
It would be advantageous if an inspection could determine whether a given defect is lithographically significant before any such further inspection is performed. By lithographically significant the inventors mean that a defect has lithographic significance in the final printed pattern. That is to say, that some defects, although present in the mask, have no significant impact on the printed pattern transferred to a photoresist layer of a substrate. For example, in some cases the defect can be so small (or on a lithographically insensitive portion of the pattern) as to be largely irrelevant. Another particularly troublesome family defect sites that are difficult to characterize are defects in so-called assist or OPC features. This problem is becoming particularly common with the increased reliance on RET (Resolution Enhancement Techniques) masks. Such masks commonly contain optical proximity correction (OPC) features and SRAFs (Sub-Resolution Assist Features). Such features although important for purposes of obtaining accurate feature representation on the final wafer may not be affected by the presence of small defects.
Thus, a lithographically significant defect is a defect that is present on the mask, but more importantly, its presence on the mask reticle can cause an effect in the lithographically transferred pattern. Such lithographically significant defects can cause problems related to circuit failures, sub optimal performance, and so on.
FIGS. 3(a)-3(i) schematically illustrate some aspects of lithographically significant defects.
FIG. 3(a) depicts, for example, the rectangular shape of an intended feature 300 that a designer wishes to print onto target surface (usually a photoresist). At small dimensions standard binary masks frequently don't do a very good job of achieving accurate photolithographic pattern transfer. However, in many cases OPC features can be used to enhance pattern transfer fidelity. FIG. 3(b) is a schematic simplified depiction of a mask pattern feature 310 that can be used to print an improved pattern onto the substrate. For example, in some pattern embodiments, OPC features 311 can be added to the corners to increase pattern transfer fidelity. FIG. 3(c) illustrates pattern 300t produced by photolithographic transfer onto the substrate. It is noted that the corners are a little rounded, but the pattern is generally very close to the design specification 300. Typically, design patterns are not expected to be perfect, but rather to conform to some pre-specified tolerance or optical design rule (ODR) specified by the designer or manufacturer. A feature in compliance with the ODR is satisfactory and not deemed to have defects. For the illustrative purposes of this patent, this transferred (or printed) feature 300t is considered to have no defects.
FIG. 3(d) depicts a mask pattern 320 similar to that of FIG. 3(b). The important difference being that there is a defect D in the mask pattern. Significantly, the inventors point out that the defect D is in one of the OPC features 321. The question is whether the defect is bad enough to have any photolithographic consequences when printed to the substrate. For the sake of this patent, in this illustration, the defects small size and presence in an OPC feature have prevented it from being printed to the substrate (See, FIG. 3(e) which shows the resultant printed feature 320t having no evidence of the defect). This defect is not generally significant and generally presents a costly waste of time to classify or repair. This defect is presented in contrast with the defect in FIG. 3(f). FIG. 3(f) depicts a mask pattern 330 generally similar to that of FIG. 3(b) with the exception being that there is a defect D in the mask pattern. Significantly, this defect is not in an OPC area and is in (for example) an optically sensitive region of the mask pattern. If the defect is large enough it will present a defect artifact when it is printed onto the substrate. FIG. 3(g) illustrates the problem when the defect D prints to the substrate. Here an observable artifact D″ of the defect D is printed. This defect is said to be photolithographically significant because it produces observable consequences when printed to the substrate.
This issue comes to rise in many situations in photolithographic processing. For example, in line printing a line feature can include several “assist” or SRAF features to enable the line to be printed robustly within process window. This phenomenon is briefly illustrated in figurative illustrations FIG. 3(h) and FIG. 3(i). FIG. 3(h) shows a mask pattern 335 used to generate a line 337 when printed onto a substrate surface. Notable are the presence of assist features 336 which do not print onto the substrate. FIG. 3(i) gives one example of a lithographically insignificant defect D. FIG. 3(i) shows a defect bearing mask pattern 339 used to generate a line when printed onto a substrate surface. In this case, the presence of defect D does not affect the final line 337 as printed. Thus, the difference between lithographically significant defects and less important defects is generally understood. The inventors point out that these examples are illustrations only and the invention, as disclosed herein, is not to be limited by such examples.
The schematic depictions of the images such as shown in FIGS. 3(c), 3(e), 3(g), and the right hand portions of FIGS. 3(h) and 3(i) are how a high NA lithographic system “sees” or reproduces and image. None of the OPC features are printed. As such, these OPC features, although present in the mask and necessary to obtain satisfactory pattern fidelity, are lithographically insignificant. High magnification tools see the OPC features just as if they were the pattern itself.
Others have attempted photomask inspection based on photolithographic significance. However, such systems have encountered a number of problems. FIG. 2 is an illustration of a common prior art inspection system 210 embodied in some prior art approaches. In such prior art systems and techniques, the optical characteristics of the illumination optics 211 are modeled on those of the stepper systems used. Additionally, the object NA 212o of the inspection system 210 is matched to a corresponding stepper in order to mimic the stepper optically. But, such optical parameters are not sufficient to provide the required magnification to enable inspection. Accordingly, a magnification optical system 213 must be employed on the imaging side in order to image the photomask onto sensors 214. Because the stepper systems all use a relatively low object NA 212o and even lower NA is required on the imaging side. Thus, the image NA 212i of optical system 213 has a very small NA. However, as a result of the changes to the optical properties of such systems, these magnification systems do not accurately model the lithographic properties of the high NA (image NA) systems (e.g., FIG. 1A) used to form patterns of the desired surfaces. Accordingly, the accuracy of measurements of photolithographic significance has been distorted by the changes in the optical system to such a degree as to be insufficient for the state-of-art lithography pattern defect detection. Thus their usefulness in photomask inspection has been severely limited. The current invention is intended to address the shortcomings of the prior by introducing new methods and apparatus approaches.
The inventors note that similar problems exist in photomask AIMS (Aerial Image Measurements Systems) review. As is known to those having ordinary skill in the art, photomask AIMS review stations are typically used to examine individual sites (i.e., individual defects or repairs done on a defect) to gauge the effect the inspected site will have on the transferred wafer pattern. Prior art photomask AIMS review systems also match the object NA of the review tool to that of the corresponding stepper used to transfer the associated pattern. Similarly, very low image NA's are used to achieved the required magnification. Not surprisingly, the prediction accuracy of such systems is unsatisfactory as the state-of-art photolithography patterning uses a higher image NA (further enhanced with an immersion liquid) than is used for the AIMS review tool. Thus improvements are need for these tools as well.
Additionally, the invention as disclosed herein present opportunities for improving wafer pattern verification techniques. Such verification is used to confirm that wafer patterns resulted from photolithographic processes produce the patterns that the designers wanted originally. Currently, the state-of-art verification technique is to perform verification on post-OPC design layout or e-beam wafer verification. In one prior art approach, a perfect design layout is used to simulate what the wafer pattern would be. The drawback of this approach is that it cannot take compensate for imperfect mask fabrication which is present in almost all cases. Accordingly, there are systematic differences between the pattern drawn in those layouts versus what is actually on the mask. Another approach uses e-beam imaging to acquire high-resolution e-beam images of the wafer pattern itself. Some drawbacks of this approach include the low speed at which such e-beam imaging is performed and the high signal-to-noise ratio for this method. In accordance with some embodiments of the invention, accurate photolithographic images acquired using the present inventive reticle inspection system can be used to predict accurately the wafer pattern, thus enabling a full-chip high-speed verification against the intended wafer pattern. Thus, the present invention offers room for improvement in the pattern verification domain as well.
The embodiments of invention present substantial advances over the existing methodologies and overcome many limitations of the existing art in photomask inspection, AIMS review and verification. These and other inventive aspects of the invention will be discussed herein below.